offset and backspacing question
#1
05-03-2013 | 10:10 AM
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Posts: 8
offset and backspacing question
Iam looking at putting 18x9 rims with a 35x12.5 tire.I leveled my h3 and i know this will work, but the question i cant find an answer to is what backpacing and offset range i should be looking at.Stock backspacing is 5.5" and has a 30mm offset.A 10mm offset would equal a 5.4 " backspace it would be .2 " closer to the brake components and tire would stick out 1.8" further.Anybody with this setup i would appreciate a response
#2
05-03-2013 | 02:32 PM
Senior Member
Joined: Sep 2009
Posts: 1,356
From: Connecticut
Don't get hung up on the Offset number. It changes with wheel width.
The important number is Backspace. As long as it's less than 5.5" then you will be OK.
So if your new wheels have 5.4" Backspace, they should be fine.
The only other thing to confirm is that the center bore is larger than 100mm diameter.
It has to be that big to clear the center hub.
The important number is Backspace. As long as it's less than 5.5" then you will be OK.
So if your new wheels have 5.4" Backspace, they should be fine.
The only other thing to confirm is that the center bore is larger than 100mm diameter.
It has to be that big to clear the center hub.
#4
05-03-2013 | 03:17 PM
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Joined: Sep 2009
Posts: 1,356
From: Connecticut
Once again, Offset means NOTHING. It's different with each wheel width.
Offset is a result of wheel width and Backspace.
No 2 wheel models are exactly alike. So you CANNOT rely on Offset to get wheel clearance right.
Backspace is a fixed number regardless of wheel width.
You need 5.5" Backspace or less to clear H3 rotors & calipers.
The smaller the Backspace number, the further your tires will stick out.
Rim width will affect how much the tires stick out too.
Offset is a result of wheel width and Backspace.
No 2 wheel models are exactly alike. So you CANNOT rely on Offset to get wheel clearance right.
Backspace is a fixed number regardless of wheel width.
You need 5.5" Backspace or less to clear H3 rotors & calipers.
The smaller the Backspace number, the further your tires will stick out.
Rim width will affect how much the tires stick out too.
Last edited by Bunger; 05-03-2013 at 03:26 PM.
#5
05-03-2013 | 03:43 PM
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Joined: Jul 2007
Posts: 12,491
From: Boat Town USA MI
It would have been better if you took a lok at this before posting in the FAQ section. 
https://www.hummerforums.com/forum/h...section-19461/
https://www.hummerforums.com/forum/h...section-19461/
#6
05-03-2013 | 06:14 PM
Senior Member
Joined: May 2012
Posts: 2,005
Finally!
I knew what backspace was but couldn't get the offset right. from bungers info I went to a tire site and got a picture of offset,bungers right,not very important. backspace pretty much tells It all,but,
"You need 5.5" Backspace or less to clear H3 rotors & calipers." is not quite right. the 5.5 is right,my finger just cleared the upper control arm,"just!" but the rotors/caliper are neatly inside the wheel, it could only hit the control arm if it was over 5.5.
in the rear , nothing is close.
Btw,when I was putting some fat rubber on an old car years ago I ordered wheels made to fit from California?. Not knowing the terminology I gave them the exact measurements from the mounting flange to the inner/outer fender and,THEY still got it wrong! by the thickness of the wheel bolt flange. I had to "adjust" the fender to fit them.
'
How to Measure Wheel BackSpace
Items required to measure wheel backspace: 
Tape measureStraight edgeWheel w/o tire (preferred)The easiest way to measure backspace is to lay the wheel face down onto the ground so the backside of the wheel is facing up. Take a straight edge and lay it diagonally across the inboard flange of the wheel. Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is backspace. The above photo shows three wheels with 2",3", & 4" backspace.
Measuring Wheel Offset
To calculate offset you'll need the following measurements:
Wheel backspaceWheel WidthWheel Center line (outboard flange to inboard flange measurement / 2) Subtract:
Wheel center line from Wheel backspace to get offset. 
If backspace is less than the wheel centerline the offset is negativeIf backspace is greater than the wheel centerline the offset is positive Tip:
To convert from inches to mm multiply by 25.4To convert from mm to inches divide
"You need 5.5" Backspace or less to clear H3 rotors & calipers." is not quite right. the 5.5 is right,my finger just cleared the upper control arm,"just!" but the rotors/caliper are neatly inside the wheel, it could only hit the control arm if it was over 5.5.
in the rear , nothing is close.
Btw,when I was putting some fat rubber on an old car years ago I ordered wheels made to fit from California?. Not knowing the terminology I gave them the exact measurements from the mounting flange to the inner/outer fender and,THEY still got it wrong! by the thickness of the wheel bolt flange. I had to "adjust" the fender to fit them.
'
How to Measure Wheel BackSpace
Items required to measure wheel backspace: Tape measureStraight edgeWheel w/o tire (preferred)The easiest way to measure backspace is to lay the wheel face down onto the ground so the backside of the wheel is facing up. Take a straight edge and lay it diagonally across the inboard flange of the wheel. Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is backspace. The above photo shows three wheels with 2",3", & 4" backspace.nuff said
Measuring Wheel OffsetTo calculate offset you'll need the following measurements:
Wheel backspaceWheel WidthWheel Center line (outboard flange to inboard flange measurement / 2) Subtract:Wheel center line from Wheel backspace to get offset. If backspace is less than the wheel centerline the offset is negativeIf backspace is greater than the wheel centerline the offset is positive Tip:To convert from inches to mm multiply by 25.4To convert from mm to inches divide
Last edited by happythree; 05-03-2013 at 08:05 PM.
#7
05-03-2013 | 07:23 PM
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Junior Member
Joined: Oct 2012
Posts: 8
thanks bunger I went to order 18x9 wheels and they wanted the offset .When you change the offset the backspacing changes.So they work together The big question was what backspacing and offset would work best. if you take the same rim a 0 offset =5" B.S. a 10mm offset = 5.4" backspace. so I should stay close to the 5.5 " B.S. thank you sorry doc olds i didnt realize i was in the FAQ section
#8
05-03-2013 | 08:16 PM
Senior Member
Joined: May 2012
Posts: 2,005
duffy,if you need to do something now,ill say what I think bunger will tell you,
FORGET the word offset. you must have 5.5 backspace,regardless of the wheel size,or less. more will hit the control arm. go out and put your finger in there and see how close it is.
backspace determines that other word that you don't need.
im guessing that bigger tires will hit inside sooner with more backspace so less is better anyway.
just get a guarantee that they do it right.
FORGET the word offset. you must have 5.5 backspace,regardless of the wheel size,or less. more will hit the control arm. go out and put your finger in there and see how close it is.
backspace determines that other word that you don't need.
im guessing that bigger tires will hit inside sooner with more backspace so less is better anyway.
just get a guarantee that they do it right.
#10
06-14-2013 | 01:37 AM
Junior Member
Joined: Sep 2008
Posts: 24
From: Colorado Springs
I knew what backspace was but couldn't get the offset right. from bungers info I went to a tire site and got a picture of offset,bungers right,not very important. backspace pretty much tells It all,but,
"You need 5.5" Backspace or less to clear H3 rotors & calipers." is not quite right. the 5.5 is right,my finger just cleared the upper control arm,"just!" but the rotors/caliper are neatly inside the wheel, it could only hit the control arm if it was over 5.5.
in the rear , nothing is close.
"You need 5.5" Backspace or less to clear H3 rotors & calipers." is not quite right. the 5.5 is right,my finger just cleared the upper control arm,"just!" but the rotors/caliper are neatly inside the wheel, it could only hit the control arm if it was over 5.5.
in the rear , nothing is close.
Method Wheels that are 18x9 come with two available back spaces, 4.5" and 5.75". Method says both fit the H3 but the -12 is used with the biggest tires so you don't rub turning all the way to lock tire wise.
If a 16" with 5.5" just clears, then an 18" has the rim an inch further out and should clear a bit more. Anyone disagree? I prefer to not have the tires sticking out as much.
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